What complications can occur in pregnancy if the Rh factor of both parents is different?

Answered by: Dr Shirish Kumar | Haematologist, WHO, Geneva

Q:My blood group is B negative, and my wife's is O positive. We have been told that there is a possibility of defects or complications in pregnancy. What is this reaction called, and what is the probability of there being problems? Is it a common problem?

A:The Rh (rhesus) factor is a protein (antigen) present on the surface of red blood cells (RBCs). As it was first discovered in the rhesus monkey, it has been so named. Individuals carrying this antigen are called Rh-positive (Rh+) while those lacking it are Rh-negative blood group type (Rh-). Rh incompatibility (or Rh disease) is a state in which a woman with Rh-negative blood group is exposed to Rh-positive RBCs leading to the formation of antibodies against this protein (called Rh antibodies). Once these antibodies are made in the mother, they cross the placenta into the developing baby’s circulation and destroy the baby’s RBCs which are Rh +.
This can occur when an Rh-negative woman is carrying an Rh-positive baby or if an Rh-negative woman receives blood transfusion containing Rh-positive cells. In either case, her immune system is exposed to the Rh antigen (which is ‘foreign’ to her body) and begins producing Rh antibodies. During pregnancy and delivery, red cells from the baby enter the mother’s circulation. If the exposure is significant, the mother gets sensitised and begins to produce antibodies. When these antibodies cross the placenta and enter the baby’s circulation, they encounter Rh + cells, which are then destroyed by these antibodies leading to anaemia and jaundice in the baby. Delivery is the usual time when sensitisation occurs and thus, first-born babies are usually unaffected, as antibodies have not yet formed in the mother. A subsequent pregnancy with an Rh-positive baby can give rise to anaemia in the baby as these antibodies destroy the baby’s RBCs. Each successive pregnancy poses a greater risk to the baby. It is postulated that even less than 1 ml of Rh-positive blood can induce antibody formation while several studies have shown that ~30% Rh-negative individuals never form antibodies despite exposure to Rh antigen.
The problem of Rh incompatibility is of clinical importance only in a pregnant woman or if blood transfusion is required. These circulating antibodies in a woman’s blood are otherwise harmless. Most institutions give human anti-D immune globulin (Rh IgG) antenatally to Rh-negative pregnant women. If a woman has been already sensitised she needs to get the antibody titres estimated and be closely monitored during pregnancy. Babies born to such mothers too may require specialised neonatal care.